Building queries using plain functions

Filtering params.ex

def index(conn, params) do
  Post
  |> filter_upcoming(params)
  |> filter_attributes(params, whitelist: ~w(title body)
  |> filter_timespan(params)
  
  # Or simplify it if it's used in multiple controllers
  Post |> filter_post_params(params)
end

def filter_post_params(query, params) do
  query
  |> filter_upcoming(params)
  |> filter_attributes(params, whitelist: ~w(title body)
  |> filter_timespan(params)
end

def filter_upcoming(query, %{"upcoming" => "true") do
  from row in query, where: row.publish_at > Ecto.DateTime.utc
end
def filter_upcoming(query, _params), do: query

def filter_timespan(query, %{"start_time" => start_time, "end_time" => end_time}) do
  # You'll need to cast the times somehow. Probably with Ecto.DateTime.cast!
  from row in query, 
    where: row.published_at > cast(start_time), 
    where: row.published_at < cast(end_time)
end
def filter_dates(query, _params), do: query

def filter_attributes(query, params, whitelist: whitelist) do
  params = Map.take(params, whitelist)
  Enum.reduce params, query, fn({attr, value}, query) ->
    from row in query, where: field(row, ^String.to_atom(attr)) == ^value
  end
end

The good thing about using regular function is that you can easily use just some of these if you want to use them in different controllers. You can also use the same function for different models if they have similar functionality (like a Post and a Tweet that both have a published_at field)